class AppServiceProvider extends ServiceProvider
{
    public function boot()
    {
        Model::extendCasts('phoneObject', function($value) {
            // First closure is the accessor closure.
            return new Phone($value);
        }, function($value) {
            // Second closure is the mutator closure.
            return $value instanceof Phone ? $value->getNormalizedValue() : $value;
        });
}

class SomeModel extends Model
{
    protected $casts = [
        'someAttribute' => 'phoneObject',
    ];
}